Exceedingly harmful.

synonyms: baneful, deadly, pernicious.

Likely to spread and cause an epidemic disease:
— A pestilential malignancy in the air.

synonyms: pestiferous, pestilential, plaguey.

Roget 657: insalubrious; unhealthy, unwholesome; noxious, noisome; morbific^†, morbiferous^†; mephitic, septic, azotic^†, deleterious; pestilent, pestiferous, pestilential; virulent, venomous, envenomed; poisonous, toxic, toxiferous^†, teratogenic; narcotic.    contagious, infectious, catching, taking, epidemic, zymotic^†; epizootic.    innutritious^†, indigestible, ungenial; uncongenial etc. (disagreeing) 24.    deadly etc. (killing) 361.    ... show more